As an example of methods for transmitting data recorded on a conventional compact disc (CD) or the like, there is a method called MOST (Media Oriented Synchronous Transfer). (Reference: Patrick Heck et al., “Media Oriented Synchronous Transfer—A Network Protocol for High Quality, Low Cost Transfer of Synchronous, Asynchronous, and Control Data on Fiber Optics”, Presented AES 103rd, 1997 September, Preprint 4551.)
Hereinafter, the conventional data transmission method according to the MOST method will be described with reference to FIGS. 11 and 12.
FIGS. 11(a) and 11(b) are diagrams illustrating a data structure of a frame included in data to be transmitted.
As shown in FIG. 11(a), according to the conventional transmission method, data are transmitted in frames, which are transmitted at 44.1 kHz, i.e., once every 22.67 msec. The data length of one frame is 512 bits. As shown in FIG. 11(b), the frame is composed of a preamble 501, a boundary descriptor 502, a synchronous channel area 503, an asynchronous channel area 504, a control frame 505, a frame control data 506 and a parity 507. Here, the cycle of the frame is not limited to 44.1 kHz but it may be 48 kHz.
Here, the preamble 501, the boundary descriptor 502 and the synchronous channel area 503 in the frame are described, while the asynchronous channel area 504, the control frame 505, the frame control data 506 and the parity 507 will not be described here because they are not directly associated with the present invention.
The preamble 501 is 4-bit data having a determined pattern, which is used by a transmission apparatus and a receiving apparatus performing transmission to detect boundaries of frames. The boundary descriptor 502 is 4-bit data, which indicates how many pieces of 4-byte block data constitutes the synchronous channel area 503 and is used to show the boundary between the synchronous channel area 503 and the asynchronous channel area 504.
The synchronous channel area 503 is data having the length of 0˜480 bits, which length is decided by the boundary descriptor 502. The synchronous channel area 503 is used for transmission of real time data such as audio. Here, the real time data is data each having time restriction and transmission of the real time data requires that the transmission delay time should be definable.
The data constituting the synchronous channel area 503 are allocated to the transmission apparatus and the receiving apparatus in byte units as time slots. That is, the data constituting the synchronous channel area 503 can use maximum 60 time slots as the synchronous channel area. The time slots which are to be used for transmission are previously allocated to the transmission apparatus and the receiving apparatus, and a group of time slots used for one transmission is defined as a logical channel. The transmission apparatus transmits data using the allocated time slots, and the receiving apparatus receives data of the allocated time slots. Using one time slot in the MOST method corresponds to transmission of data at a transmission rate of 352.8 kbps. To be more specific, data of a compact disk (CD) having a transmission rate of 1.4112 Mbps can be transmitted by using 4 time slots of the synchronous channel area 503.
FIG. 12 is a diagram for explaining a method for transmitting digital audio data using the conventional synchronous channel.
In this figure, contents of time slots in respective frames 1 and 2 which are used by L and R channels for transmitting digital audio data are shown. 16-bit audio samples 1 and 3 in the frames 1 and 2 are transmitted by the L channel and 16-bit audio samples 2 and 4 are transmitted by the R channel, respectively.
According to the conventional transmission method, a method for transmitting 2-channel audio data which are recorded on a CD or the like is determined, while a data transmission method for transmitting multi-channel audio data which are recorded on a DVD-Video or a DVD-Audio, or audio data having a higher sampling frequency or a longer quantization word length is not determined and thus the multi-channel data cannot be transmitted.
In addition, according to the conventional transmission method, when data are encrypted for copyright protection typified by DTCP (Digital Transmission Content Protection Specification), there are following two problems. The first one is that when commonly used encryption in 8-byte units is performed in the transmission apparatus, data of each frame according to the conventional transmission method is not always a multiple of 8 bytes and thus the encryption is not completed within the frame. The second problem is that although copyright protect information is supposed to be transmitted according to the DTCP, its transmission method is not determined in the conventional transmission method.